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Donald Trump hasn't let many insults slide this campaign season - except when they come from Hillary Clinton's running mate.

The Republican presidential nominee has all but ignored the constant barrage of attacks from Virginia Sen. Tim Kaine, who has assumed the traditional attack-dog role of a vice presidential candidate with gusto, but little notice.

It could be a sign of a newfound discipline by Trump. Or it could that Kaine's attacks don't rise above the din of the daily news cycle. Or, perhaps, that Trump just doesn't care.

In recent weeks, Kaine has asserted Trump has a "bizarre fascination'' with dictators and Vladimir Putin, said he "choked'' during his trip to Mexico and even associated Trump's values with those of the Ku Klux Klan. Criticizing Trump as unfit to be president has become a central theme of Kaine's public remarks as he works to show the Democratic ticket as more qualified to run the country. In an attempt to paint Trump as reckless, Kaine often repeats Clinton's line that Trump is a candidate you can "bait with a Tweet.''

Trump Booed Leaving New York Times

President Elect Donald Trump is booed as he walks through the lobby of The New York Times Building after a 75-minute meeting with Times journalists. The lobby of the Times building is open to the public, and a large crowd had gathered by the time he departed.

(Published Tuesday, Nov. 22, 2016)

But if it's bait Kaine is casting, Trump isn't biting. Trump's only recent retort - ia a Tweet - came after a recent Kaine appearance on a morning news show. He didn't even mention Kaine by name.

"Crooked Hillary's V.P. pick said this morning that I was not aware that Russia took over Crimea. A total lie - and taken over during O term!'' Trump tweeted, with "O'' referring to President Barack Obama.

Ignoring the barbs of the opposing side's vice presidential candidate is common practice among presidential candidates who don't want to be seen punching below their weight. Presidential hopefuls typically attack only their main opponent, but Trump is an uncommon candidate whose rise is largely marked by his willingness to buck tradition. And Trump has not held back from attacking those he believes have slighted him, whatever their prominence. His many targets have included many other politicians, individual journalists, and celebrities.

In recent weeks Trump has made more of an effort to attack GOP Sen. Jeff Flake of Arizona and morning TV news host Mika Brzezinski than he has Kaine.

"Just heard that crazy and very dumb (Mika) had a mental breakdown while talking about me,'' Trump tweeted recently.

Still, Trump has been showing more restraint lately than he has during much of his campaign. When Clinton abruptly left a 9/11 anniversary ceremony Sunday and her campaign revealed that she had been diagnosed with pneumonia, Trump said that he hopes she feels better.

Trump Takes Meetings at His New Jersey Golf Club

President-elect Trump interviewed more than a dozen candidates for his administration at his New Jersey golf club over the weekend, including Mitt Romney, Rudy Guliani, Chris Christie and Kansas secretary of state Kris Kobach, who has been tough on immigration, and others.

(Published Monday, Nov. 21, 2016)

Indiana Gov. Mike Pence, Kaine's counterpart on the Republican ticket, did punch back at Kaine on Saturday for calling the GOP ticket unpatriotic in their praise of Russian President Vladimir Putin.

"Well, Tim, when Donald Trump and I said that the small and bullying president of Russia was a strong leader on the world stage, that wasn't an endorsement of Putin,'' Pence said, going on to say it was an "indictment'' of Obama.

Trump's campaign has paid particular attention lately to Kaine's home state with a heavy TV ad buy. Once viewed as a crucial swing state, Virginia has been increasingly leaning in favor of Clinton this campaign cycle. Kaine is the state's former governor and has a strong following among the state's Democratic base. But his support is not strong enough to make Trump hesitant to attack him.

The Clinton campaign suggests Trump may be keeping mum because his past attempts to criticize Kaine haven't been effective. In late July, Trump wrongly identified Kaine as the governor of New Jersey as he attempted to attack his record. When Clinton first named Kaine her running mate, Trump's campaign labeled him "Corrupt Kaine.'' The Republican National Committee has also called him "Weathervane Kaine,'' charging him with flip-flopping. But none of the attacks have been used with regularity or proved to have the same staying power as Trump's nickname for Clinton, "Crooked Hillary.''

Asked along the ropeline in Norfolk, Virginia, if he was surprised Trump wasn't responding more to him, Kaine demurred.